1. Field of the Invention
This invention relates to a method of making a substrate used for a magnetic recording medium such as a video tape, an audio tape, a computer tape, a floppy disk, an electric still video floppy disk, or a magneto-optic disk.
2. Description of the Prior Art
As the recording density of magnetic recording media is increased, their substrates are required to have smoother and more slideable surfaces. Also for polyethylene terephthalate films, which are the main substrates used for magnetic recording media, in order to ensure the quality of high density magnetic recording, the film surface texture should be such that finer protrusions having more uniform heights are arrayed at a higher density than in films for packaging materials and electric insulation.
In recent years, attempts have been made to make a high density magnetic recording medium by applying a metal magnetic material onto a substrate together with a binder or by forming a thin ferromagnetic film of a metal on a substrate by vacuum deposition or sputtering. The surface texture of the substrate used for this purpose must satisfy strict requirements. Specifically, the surface texture must be such that the surface unevenness is of a very minute order, so minute that it cannot be accurately measured with a stylus type surface texture measuring instrument, and the heights of protrusions and recesses are uniform.
However, the surface texture of the film used for the magnetic recording medium can be improved only to a limited extent for the reasons described below. Namely, when the surface texture of the film is good or smooth, the frictional resistance of the film with respect to conveying rollers becomes large, and the film creases or zigzags in the film forming winding step. Further, the frictional resistance between the film surfaces themselves also increases, so that the form of the wound film roll is distorted.
In the case of a film for a coating type agnetic recording medium, a desired surface texture can be obtained by high-density filing of minute filler grains having uniform grain sizes. However, as the surface smoothness of the film becomes high, the yield in the film forming step decreases inevitably due to the aforesaid handling difficulty. On the other hand, in the case of a film for a thin metal film type magnetic recording medium, a high filling density and height uniformity are required for protrusions having heights of a more minute order (50 .ANG. to 1,000 .ANG.).
On the other hand, a method of forming a discontinuous film comprising a water-soluble high-molecular weight compound and fine grains on a film surface is disclosed in Japanese Unexamined Patent Publication Nos. 59(1984)-84927 and 59(1984)-121631. Also, Japanese Unexamined Patent Publication No. 60(1985)-27539 discloses a method of forming a coating film having many minute protrusions by applying a mixture of cross-linkable polyurethane, silicone and fine grains onto a film surface.
However, with these disclosed methods, slideability cannot be maintained substantially high, and the yield in the film forming step decreases to a value markedly lower than in the substrate for the coating type magnetic recording medium. Also, a magnetic recording medium made by forming a thin metal film on the obtained substrate film cannot exhibit high running characteristics.
As mentioned above, the manufacture cost of the substrate for the high density coating type magnetic recording medium and the substrate for the thin metal film type magnetic recording medium have heretofore been very high. Particularly, in the case of the substrate for the thin metal type magnetic recording medium, no technique for imparting high running characteristics has been established.